Identification of layered cloud occurrences from the Lidar In-Space Technology Experiment and advanced very high resolution radiometer imagery Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/qb98mj812

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  • Realistic assessment of the vertical distribution of clouds, particularly the occurrence of multi-layered systems, is critical for accurate calculations of radiative transfer in general circulation models. Such information is also useful in the design and improvement of satellite retrieval techniques. Current methods for retrieving cloud properties from satellite data assume that the clouds reside in single-layered systems. These methods are not expected to be successful for multi-layered systems. Attempts to specifically address the question of cloud layering have thus far been limited, due in part to the difficulties of inferring vertical cloud structure from either surface or satellite data. In situ observations, such as those provided by aircraft, are available only for localized regions and are limited in time. This study uses data from a lidar instrument flown onboard the space shuttle and satellite imagery data to identify the frequencies of occurrence of layered cloud systems at different spatial scales over various regions of the globe. The Lidar In-Space Technology Experiment (L1TE) was flown on Space Shuttle Discovery in September 1994 and provided global-scale, high vertical resolution profiles of the earth's troposphere and lower stratosphere. Analysis of the LITE observations requires distinguishing clouds residing in organized, well-defined layers from clouds that are distributed in altitude throughout the troposphere. The analysis employs a histogram technique in which peaks having some critical number of observations are considered to correspond to observations belonging to well-defined cloud layers. Advanced Very High Resolution Radiometer (AVHRR) data for the 11-day duration of the LITE mission are analyzed using the spatial coherence method. This method identifies regions of locally uniform emission which are associated either with cloud-free pixels or with overcast pixels corresponding to clouds in a single layer at a well-defined altitude. The number of layers present is determined by the number of overcast radiances associated with pixel arrays exhibiting locally uniform emission within the region. Layer statistics are compiled for the Pacific, Atlantic, and Indian Oceans and the North and South American, African, European, Asian, and Australian continents using horizontal scales of 60 and 250 km. The results indicate a strong dependence on the spatial scale chosen for the analysis, with two- and three-layered systems more prevalent at the 250-km scale. Analysis of cloud-top altitudes from LITE and AVHRR show that low-level cloud systems comprise the majority of the observations over both ocean and land.
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